1. Field of the Invention
The present invention relates to a method for fabricating solar cells having integral collector grids in heterojunction as well as Schottky barrier devices. More particularly, the present invention relates to photovoltaic devices of the heterojunction and Schottky barrier type which possess a metallic collector grid system as an integral part of a conductive transparent mixed metal oxide on the photovoltaic substrate.
2. Description of the Prior Art
Solar cells utilize heterojunctions such as Ga.sub.1-x Al.sub.x As-GaAs, ZnSe-GaAs, GaP-Si or Schottky barriers such as those in which a thin film of metal such as platinum, gold, silver or the like is deposited on a semiconductor substrate are well known. nZnSe-pGaAs heterojunction devices have been prepared by vapor growth as well as solution techniques. However, the conversion efficiency of the device is low because of the high doping levels of GaAs required and because of the high series resistance attributable to the high resistivity of ZnSe. GaP-Si heterojunction devices have been prepared, but a major problem is that when the devices are cooled from growth temperature, cracks develop in the devices because of the stress caused by thermal expansion differences between GaP and Si.
Since silicon represents the most highly developed semiconductor technology, it is the most promising basic material for the fabrication of practically useful solar cells. Heterojunction Si solar cells are known in which SnO.sub.2, In.sub.2 O.sub.3, CdS or GaP is deposited on n- or p-Si. Efficiencies of up to 10% can be achieved with SnO.sub.2 deposited on n-type Si, close to 6% with CdS deposited on p-type Si and as high as 12% with In.sub.2 O.sub.3 deposited on p-type Si. While the heterojunction Si solar cell devices have no surface dead layer which is caused by low-lifetime, highly doped, diffused regions, and while processing of the devices are simpler as well as amenable to continuous-flow manufacture, it is critically important to control the junction interface to achieve optimum performance.
Schottky barrier silicon cells are potentially effective as solar cell devices because they feature good short wavelength response characteristics and they do not require processing at elevated temperatures. Moreover, efficiencies of up to 8% have been obtained with devices containing aluminum as the barrier metal and even higher efficiencies (up to 8.6%) with chromium as the barrier metal.
Recently, potentially valuable types of heterojunction and Schottky barrier type solar cells have received increased interest, and these cells involve the use of transparent conducting glasses such as In.sub.2 O.sub.3, SnO.sub.x, ZnO and the like as coatings on solar cell devices. It has been suggested to use such materials in the manufacture of heterojunction devices with p-type semiconductors or Schottky barriers with n-type semiconductors. These metal oxide materials have the desirable characteristic of having high transparency which is usually above 90% at layers of several thousand A to about one micron in thickness and their resistivities in the form of a sheet can be very low. Such metal oxides can also be applied by evaporation, spraying and sputtering techniques onto a suitable substrate. Moreover, the metal oxides are potentially very economical from cost and energy consumption viewpoints. In fact, some solar cell devices have been manufactured of both the heterojunction and Schottky barrier type in which SnO.sub.2 is deposited on Si and GaAs substrates. However, the efficiencies of the devices are poor, i.e., about 1%, probably because of the poor quality of the SnO.sub.2 films or because of detrimental energy barriers at the interface of the devices.
Anderson in Applied Physics Letters, 27(12), (1975), 691-693, has described heterojunction solar cells in which a thin insulating layer such as SiO.sub.2 is disposed at the material interface. The reference shows heterojunction photovoltaic cells which have the structure, n-In.sub.2 O.sub.3 /I/p-Si in which an intermediate SiO.sub.2 layer exhibits insulating characteristics. It has been found that the intermediate SiO.sub.2 layer increasingly suppresses photocurrent at increasing illumination intensity and at increasing interfacial thicknesses. Although the presence of such an intermediate layer increases the conversion efficiency of solar cells at low illumination intensities, the fact that the SiO.sub.2 layer suppresses the photocurrent generated by the device obviates its use in systems which use appreciable solar concentration.
Wang et al, Proceed. of the 12th Photovoltaic Specialists Conference, Baton Rouge, La., Nov. 15-18 (1976), have disclosed SnO.sub.2 -Ge and SnO.sub.2 -GaAs heterojunction devices which exhibit photovoltaic characteristics. Moreover, conducting glass-silicon heterojunction solar cell devices in which SnO.sub.2, In.sub.2 O.sub.3 and mixtures of In.sub.2 O.sub.3 and SnO.sub.2 are used as conducting glasses are known. The oxide semiconductor coatings possess good electrical conductivity and high optical transparency. Because the index of refraction of the oxide films falls between that of air and the semiconductor substrate, the oxide semiconductor not only serves as a part of the heterojunction, but also as an antireflection coating. The oxide semiconductor heterojunction devices also have the potential advantages over conventional homojunction solar cells such as low temperature diffusionless fabrication, shallow junction depth and batter radiation resistance.
Another known photovoltaic device consists of a silicon substrate provided with a conductive, transparent layer of SnO.sub.2. Usually, the tin oxide coating is deposited on the substrate by sputtering or evaporation of tin onto the substrate and then oxidizing the metal layer by some convenient technique. However, while a number of heterojunction and Schottky barrier devices are known, which possess photovoltaic activity including a number which possess a transparent conductive metal oxide coating, and which have surface collector grids in contact with the active metal oxide layer, a need continues to exist for a method by which photovoltaic devices can be fabricated in which the transparent, conductive metal oxide coating of such devices is not only a conductive sheet, but also possess bulk metal collector grids within the oxide layer in intimate contact with the initial oxide layer in contact with the semiconductor substrate of the photovoltaic device.